Chemistry 068, Chapter 8

Moles and Chemical Equations Chemical equations can be viewed in terms of molecules or in terms of moles. The coefficients in the chemical equations give the ratios of reactants and products. These ratios are the ratio of molecules at the microscopic (single molecule) level and the ratio of moles at the macroscopic (real world) level. These ratios are useful in problem solving.

Moles and Chemical Equations (Cont’d) For example: 2A + B 2  2AB –Ratios will be the same at both single molecule and mole level. –2 molecules A and 1 molecule B 2 makes 2 molecules AB. –Likewise for moles, 2 moles A and 1 mole B 2 makes 2 moles AB. –This ratio will be consistent even if the number of moles or molecules varies.

Moles and Chemical Equations Problems Consider the reaction N 2 + 3H 2  2NH 3 –How many moles of NH 3 are formed when 1mol N 2 and 3mol H 2 react? –How many moles of NH 3 are formed when 2.3mol N 2 and 6.9mol H 2 react? –How many moles of N 2 and how many of H 2 are needed to produce 3 moles of NH 3 ?

Balanced Chemical Equations and Conservation of Mass Recall that the law of conservation of mass requires that the total mass of products and reactants in a chemical reaction must be equal. –This is because the type and number of each atom is the same before and after a reaction.

Stoichiometry Stoichiometry calculations use chemical equations to determine quantitative relationships between products and reactants. –In practice, you will often need to go back and forth between moles and grams. –You can use the coefficients in a balanced chemical equation as the mole to mole ratio between substances in the reaction.

Stoichiometry Problems Consider the following reaction: HNO 2  NO + HNO 3 + H 2 O –Is the reaction correctly balanced? –What mass of HNO 2 is needed to produce 5.0moles of NO? –How many moles of HNO 3 would be produced from the same mass? –How many moles of H 2 O?

Stoichiometry Problems (Cont’d) 3HNO 2  2NO + HNO 3 + H 2 O –How many molecules of H 2 O would be produced by 25.0g of HNO 2 ? –How many of NO and HNO 3 ?

Stoichiometry Problems (Cont’d) 3HNO 2  2NO + HNO 3 + H 2 O –How many grams of NO, HNO 3, and H 2 O are produced when 50.0g of HNO 2 react fully?

Stoichiometry Problems (Cont’d) 3HNO 2  2NO + HNO 3 + H 2 O –What mass of HNO 2 is needed to produce 15.0g of NO? –What mass of HNO 3 and H 2 O would be produced?

The Limiting Reagent Concept Many times the amounts you work with won’t be in perfect ratios according to your chemical equation. In such cases you will run out of one reactant before you do the other – the reactant you run out of is the limiting reagent.

The Limiting Reagent Concept (Cont’d) The sandwich analogy. –Suppose you make a ham sandwich by using two slices of bread and one slice of ham. –You would write the following “equation”: 2Bread + 1Ham  1Sandwich

The Limiting Reagent Concept (Cont’d) You need twice as much bread as ham to make sandwiches. If you didn’t have your supplies in just the right ratio you’d have either bread or ham left over. Reactions work the same way.

The Limiting Reagent Concept (Cont’d) Since you use up reactants at different rates (coefficients) you may run out of the one you actually have more of to start with. When a reactant is listed as being “in excess” it means there is much, much, more of it than any other reactant. –This means that it is not the limiting reagent – another reactant is limiting.

The Limiting Reagent Concept (Cont’d) For example, consider the following reaction. 2A + B 2  2AB In each of the following cases identify the limiting reagent and determine how many moles AB you produce. –2.0mol A, 1.0mol B 2. –3.0mol A, 2.0mol B 2. –3.5mol A, 1.2mol B 2. –Excess A, 1.0mol B 2. –1.0mol A, excess B 2.

Limiting Reagent Problems Consider the following reaction: 2NaOH(aq) + Cl 2 (g)  NaCl(aq) + NaClO(aq) + H 2 O(l) –If 1.23mol NaOH reacts with 1.47mol Cl 2, which is limiting? How much of each is there after the reaction? –If 2.47mol NaOH reacts with 1.32mol Cl 2 ?

Limiting Reagent Problems (Cont’d) Consider the following reaction: 2NaOH(aq) + Cl 2 (g)  NaCl(aq) + NaClO(aq) + H 2 O(l) –If 54.0g NaOH reacts with 42.0g Cl 2, which is limiting? How much of each is there after the reaction? –If 19.0g NaOH reacts with 113g Cl 2 ?

Percent Yield The theoretical yield is the maximum amount of product that could be produced from a given amount of reactants assuming no losses. Sometimes a reaction doesn’t give the full amount predicted as the theoretical yield. –The actual amount of product produced is called the actual yield or experimental yield. –Sometimes it is simply called the yield.

Percent Yield (Cont’d) In fact, real reactions rarely produce the full amount predicted by the theoretical yield. –Product can be lost during purification or during transfer from one container to another. –Side reactions can also occur that create different final products. We define the % Yield as: 100%*(actual yield)/(theoretical yield) –Should be less than or equal to 100%.

Percent Yield Problems Consider the following reaction: 2SO 2 + O 2  2SO 3 –What is the theoretical yield, in grams, of SO 3 if 20.0g of SO 2 with 32.0g of O 2 ? –What mass would be produced if there was only a 80.0% yield? –What would the % yield be if only 10.0g were produced?

Exothermic vs. Endothermic Phase Changes An endothermic phase change requires an input of energy. –For example, melting or boiling. An exothermic phase change releases energy. –For example, freezing or condensation. Heat is the type of energy used in either sort of change of state.

Heat Heat is the energy flow into or out of a system due to a temperature difference between a thermodynamic system and its surroundings. It is given the symbol Q. Heat can only flow between systems in thermal contact. Heat flows to attain thermal equilibrium (equal temperature). –Thus it flows from high to low temperature.

Enthalpy Enthalpy, given the symbol H, is an extensive (depends on amount) property of a substance that can be used to obtain the heat absorbed of released as part of a chemical reaction. Enthalpy is a state function and as such depends only on its present state (such as temperature or pressure) rather than the path used to get there. The heat and enthalpy change of a process are usually equal. –Thus, in general, the terms heat and enthalpy are used interchangeably.

Enthalpy of Reaction Is the change in energy for a given chemical reaction at a given temperature and pressure. Usually you are interested in the change in enthalpy for a reaction, called a delta, denoted by ΔH. ΔH = ΔH Products – ΔH Reactants Remember, enthalpy is independent of path. Enthalpy is related to heat by: ΔH = q p Where q p is heat at constant pressure.

Thermochemical Equations Thermochemical equations are chemical equations that include an enthalpy of reaction. The enthalpy of reaction is written directly after the chemical equation. For example: A(g) + 2B(s)  AB 2 (s); ΔH = -450kJ

Manipulating Thermochemical Equations – Stoichiometry and Heat of Reaction The ΔH for a reaction assumes that the listed moles react. More or less moles will change ΔH accordingly. Reversing a reaction reverses the sign of ΔH. If twice as many moles react you will have twice as much energy released. If three times then three times the energy, and so forth.

Manipulating Thermochemical Equations – Stoichiometry and Heat of Reaction (Cont’d) For example, consider the following reaction: A + 2B  AB 2 ; ΔH = -50.0kJ If 1 mole A and 2 moles B react, then ΔH = If twice as many moles react, ΔH = - 100kJ. The reverse reaction would be: AB 2  A + 2B; ΔH = +50.0kJ

Heat of Reaction Problems Consider the following reaction: A + 2B  AB 2 ; ΔH = -50.0kJ How much heat is evolved when 0.050mol of A reacts with 0.10mol B? How much heat is evolved when 1.2mol of A reacts with 1.2mol B? Is there a limiting reagent?

Heat of Reaction Problems (Cont’d) Consider the following reaction: 2H 2 (g) + O 2 (g)  2H 2 O(g); ΔH = -484kJ How much heat is evolved when 10.0g hydrogen reacts with excess oxygen?